In a high intensity discharge (HID) lamp, a medium-to-high pressure ionizable gas, such as mercury or sodium vapor, emits visible radiation upon excitation typically caused by passage of current through the gas. In HID lamps, as originally constructed, this excitation was produced by causing a discharge current to flow between two electrodes. However, a major cause of early electroded HID lamp failure has been found attributable to at least two inherent operational characteristics of such lamps. First, during lamp operation, sputtering of electrode material onto the inner surface of the lamp envelope is common and impedes optical output. Second, thermal and electrical stresses often result in electrode failure.
Electrodeless HID lamps do not exhibit these life-shortening phenomena found in electroded HID lamps. In one class of electrodeless HID lamps, an arc discharge is generated by establishing a solenoidal electric field in the gas. In particular, a solenoidal electric field is created by the varying magnetic field of an excitation coil. Current flows through the gas, thereby producing a toroidal arc discharge. Advantageously, this class of electrodeless HID lamps generally exhibits higher efficacy than standard electroded HID lamps.
The excitation coil of an electrodeless HID lamp surrounds the arc tube. As a result, the coefficient of electromagnetic coupling between the coil and the solenoidal discharge is relatively low, typically in the range from 0.2 to 0.4. Therefore, in order to produce a predetermined discharge current in the arc tube, an even larger current is required in the coil. The relatively large coil current results in resistive losses in the coil that can have a significant deleterious effect on efficiency of the overall HID lamp system. Moreover, as the temperature of the excitation coil increases, coil resistance increases. Hence, to increase efficiency of an electrodeless HID lamp system, heat resulting from coil resistive losses and from convection from the hot arc tube to the coil must be removed by an effectual method of heat sinking. Furthermore, although improvements in heat sinking are desirable for HID lamps, such improvements must not interfere appreciably with the visible light output of these lamps.
In accordance with the foregoing, to maximize efficiency of an HID lamp, the degree of coil coupling between the magnetic field and the arc discharge must be maximized. Since the degree of coupling increases with frequency, electronic ballasts used to drive HID lamps operate at high frequencies in the range from 0.1-20 MHz, exemplary operating frequencies being 13.56 and 6.78 MHz. These exemplary frequencies are within the industrial, scientific, and medical band of the electromagnetic spectrum in which moderate amounts of electromagnetic radiation are permissible; and such radiation generally is emitted by an electrodeless HID lamp system. Disadvantageously, at these high frequencies, switching losses associated with the charging and discharging of the parasitic capacitances of the power switching devices of an electronic ballast are generally high. Fortunately, however, a zero-voltage, i.e. lossless, switching technique, as described in commonly assigned, copending U.S. patent application of S. A. El-Hamamsy and G. Jernakoff, Ser. No. 454,614 filed Dec. 21, 1989, now allowed, may be used to improve the efficiency of the ballast. The El-Hamamsy and Jernakoff patent application is hereby incorporated by reference.
While operation of the ballast at the resonant frequency of the load circuit maximizes power output, operation at a frequency slightly lower than the resonant frequency of the load circuit maximizes ballast efficiency. Hence, for maximum efficiency, operation is slightly "off" resonance, and a specific ballast load amplitude and phase angle are required. To this end, the impedance of the ballast load, including that of the arc discharge as reflected into the ballast load, must be matched to the required ballast load resistance and phase angle. As described in commonly assigned, copending U.S. Pat. No. 4,910,439 of S. A. El-Hamamsy and J. M. Anderson, issued Mar. 20, 1990 and hereby incorporated by reference, a network of capacitors is generally used for impedance matching. According to the cited patent application, a suitable network includes a pair of capacitors connected in parallel with the excitation coil. Advantageously, these parallel connected capacitors have large plates that are also used to dissipate heat generated by the lamp coil and arc tube, i.e., for heat sinking.
Although the hereinabove described parallel capacitance has been found to be useful in matching the resistive component of the ballast load impedance, it has been determined that a series component of capacitance is needed to obtain the proper phase angle. A capacitance in series with the excitation coil must be capable of carrying large currents at the operating frequency of the lamp and be able to withstand high peak voltages applied thereto. Moreover, the conductive layers of standard RF capacitors, such as multilayered ceramic capacitors and RF transmission capacitors, are very thin and, therefore, have limited current carrying capability. Hence, to meet the current requirements with such RF capacitors, several standard valued RF capacitors must be connected in parallel. Such a configuration is usually bulky. Moreover, these RF capacitors are expensive because the manufacturing process is relatively complex and slow. Vacuum capacitors are also capable of handling the aforementioned voltage and current requirements, but are likewise too expensive, in addition to being too large, for widespread commercial applications. Thus, it is apparent that to maximize efficiency by matching the required ballast load impedance, the number of ballast circuit elements increases. Disadvantageously, as the number of circuit elements increases, so do the number of electrical leads and connections, resulting in more resistive losses. Moreover, electrical leads have parasitic inductances associated therewith which may introduce additional resonances into the ballast load circuit, as described in commonly assigned, copending U.S. patent application of S. A. El-Hamamsy, R. J. Thomas, and J. C. Borowiec , Ser. No. 454,549, filed Dec. 21, 1989, which patent application is hereby incorporated by reference. An additional resonance resulting from a parasitic inductance introduces waveform distortion and increases power dissipation, thereby reducing efficiency.